This review evaluates the effects and importance of disturbance
and climate change on plant community dynamics in the United
States/Mexico borderlands region. Our primary focus is on knowledge
of physiognomic-level change in grasslands and woodlands of southeastern
Arizona and southwestern New Mexico. Changes in vegetation physiognomy
have broad implications for management and land use in the borderlands.

Historically, livestock grazing and fire were the dominant
disturbances in borderland ecosystems. Livestock grazing accelerates
the rate of establishment and growth of woody plants (notably
mesquite) in the borderlands region. However, the current role
of livestock grazing in this region may be considerably less
important than the historic role. Excluding livestock from grasslands
now would not prevent, and may not delay, a substantial increase
in woody plant abundance. In general, frequent fires are detrimental
to woody plants and beneficial to Lehmann lovegrass. Although
fires favor herbaceous plants at the expense of woody plants,
woody plant dominance on many sites precludes the deliberate
application of fire without first using chemical or mechanical
treatments.

Climate change is perhaps the most critical factor facing
the current generation of land managers who are concerned about
the future condition of borderlands ecosystems. Rising concentrations
of CO2 and other trace gases have the potential to replace grazing
and fire suppression as important regulators of vegetation change.
Increased atmospheric CO2 concentrations may contribute to increased
abundance of woody plants in borderlands grasslands, particularly
if these increases are accompanied by shifts in seasonal precipitation
or other climatic factors.

Much of the knowledge garnered about borderlands plant communities
is derived from descriptive research such as comparative studies
and "natural" experiments. This research has been valuable
for documenting changes in vegetation and identifying candidate
explanations for observed changes. However, because descriptive
research is generally inappropriate for testing hypotheses, we
outline and describe a strategy for hypothesis-testing and provide
recommendations for future research in the borderlands. Implications
for management are also outlined in this paper.

Keywords: disturbance, climate change, experiments,
fire, grazing

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